LaB₆-ZrB₂ Eutectic Solidification Characteristics And Properties With Integrated Structure And Function

Lanthanum hexaboride(LaB₆)single crystal has been widely used as an excellent thermionic electron cathode material due to its low work function,high melting point,low vapor pressure at high temperature and good resistance to ion bombardment,However,LaB₆has a high brittleness and is easy to break in the process of hollow cathode processing.Therefore,Me B₂ with high elastic modulus was added into LaB₆ to form LaB₆-ZrB₂eutectic composite.In combination with spark plasma sintering technology and optical zone melting technology,eutectic composite materials with monocrystalline matrix were prepared.The influences of process parameters on the microstructure,mechanical properties and thermal emission properties of the materials were systematically investigated as follows:LaB₆-ZrB₂ eutectic composites with(100)single crystal matrix were successfully prepared at different solidification rates.When the solidification rate increases gradually from 20 to 300mm/h,the regularity of the fiber remains unchanged,the diameter and eutectic spacing of the fiber gradually decrease.The relation between the solidification rate and eutectic spacing is consistent with the relation between?R^0.42=6.42?m^1.42/s^0.42.At a given solidification rate,the eutectic spacing is a range,and with the increase of solidification rate,the distribution range of eutectic spacing becomes narrower and narrower.With the increase of solidification rate,both vickers hardness and fracture toughness of LaB₆(100)-ZrB₂ eutectic composites increase first and then decrease,and reach the maximum value at V=100 mm/h.However,vickers hardness presents weak anisotropy.The highest vickers hardness on the cross section is 23.14 GPa,and the highest vickers hardness on the longitudinal section is 23.06 GPa The fracture toughness of the composite material presented strong anisotropy.In the cross section,the highest fracture toughness of the composite material was 4.72 MPa.m^1/2;in the longitudinal section,the average fracture toughness in the direction of parallel fiber growth was 4.31 MPa.m^1/2;and the fracture toughness perpendicular to the direction of fiber growth was 5.16 MPa.m^1/2.The bending strength of high temperature shows that as the temperature increased,the bending strength increased first and then decreased.when T=1200?,the highest bending strength is 679 MPa.The mechanical properties of composite phases were studied by nano indentation technique.It was found that when the load was 40 m N,the average Berkovich hardness values of LaB₆ matrix and LaB₆/ZrB₂ interface were 27.98 GPa and 27.02 GPa,respectively,and the hardness values of composite materials decreased with the increase of maximum indentation depth.The thermal emission properties of the composites were studied by using diode theory.The current density of the composites increased with the increase of applied voltage or temperature.Under certain conditions of applied voltage and temperature,with the increase of solidification rate,the thermal emission current density of the composite increases first and then decreases.At V=20 mm/h,the composite has the highest thermal emission characteristics:under the conditions of U=1 k V and T=1873K,the thermal emission current density is 19.4 A/cm²,and the average effective escape work is 2.84 e V.LaB₆-(Zr,V)B₂ eutectic composites were successfully prepared by spark plasma sintering The effect of temperature on the density and microstructure of materials was investigated.found that with the increase of sintering temperature,the density increased gradually,when the sintering temperature is 1800?,its density is the largest,the value is 92.4%,On this basis,high quality LaB₆-(Zr,V)B₂ eutectic composites were prepared by optical zone melting technology.It was found that with the change of solidification rate,the microstructure of the eutectic composites presented a variety,mainly due to the rod-like and lamellar mixed structure of LaB₆-VB₂.